THOFURYL)DIMETHYLSILYL]ALLYL]-1,3,2-DIOXABOROLANE, AN IMPROVED CHIRALREAGENT FOR THE ANTI ALPHA-HYDROXYALLYLATION OF ALDEHYDES - APPLICATION TO THE ENANTIOSELECTIVE SYNTHESIS OF (-)-SWAINSONINE

Diisopropyl tartrate-modified (E)-[gamma-(furyldimethylsilyl)allyl]bor onates 10 and 11 were designed for the enantioselective synthesis of s ubstituted anti-3,4-dihydroxy-1-butenes 9 via the anti-alpha-hydroxyal lylation of aldehydes. The reactions of aldehydes with 10 and 11 provi ded furyldimethylsilyl-substituted anti-1,2-silanols 12 and 13 with go od enantioselectivity (74-82% ee), and the silanols were oxidized to t he corresponding anti-1,2-diols 9 by a modified Fleming-Tamao oxidatio n protocol. The high reactivity of the (menthofuryl)silane unit toward electrophilic substitution allowed complete protodesilylation of ment hofuryl-substituted silanols 13 and subsequent oxidation to diols 9 in a one-pot operation without competing protodesilylation of the allyls ilane unit. However, a two-step protocol was required for the protodes ilylation-oxidation of the less reactive 2-(5-methyl)-asymmetric react ions with three chiral aldehydes 23-35 (80 to >92% de, with the except ion of the mismatched reactions with aldehyde 25, which were essential ly unselective). However, [[[2-(5-methylfuryl)]dimethylsilyl]ally]boro nate 10 could only be synthesized in less than or equal to 15% yield, and oxidations of the 2-(5-methylfuryl)-substituted silanols 12 were l ower-yielding than oxidations of the corresponding menthofuryl-substit uted silanols 13. Thus diisopropyl tartrate-modified )-[gamma-[(mentho furyl)dimethylsilyl]ally]boronate 11 proved to be the more useful of t he two reagents. As a demonstration of the utility of 11 in the synthe sis of polyhydroxylated natural products, this new reagent was used in a brief and highly stereoselective synthesis of the naturally occurri ng alkaloid (-)-swainsonine (42).